1) Add unit test for groupBy query with dimension that doesn't exist

2) Fix group by query to work with new unit test, fixes #232

server/src/main/java/com/metamx/druid/query/group/GroupByQueryEngine.java

@@ -50,6 +50,7 @@ import org.joda.time.Interval;
 
 import javax.annotation.Nullable;
 import java.nio.ByteBuffer;
+import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
@@ -276,7 +277,7 @@ public class GroupByQueryEngine
 
     private final List<DimensionSpec> dimensionSpecs;
     private final List<DimensionSelector> dimensions;
-    private final String[] dimNames;
+    private final ArrayList<String> dimNames;
     private final List<AggregatorFactory> aggregatorSpecs;
     private final BufferAggregator[] aggregators;
     private final String[] metricNames;
@@ -295,14 +296,14 @@ public class GroupByQueryEngine
       delegate = Iterators.emptyIterator();
       dimensionSpecs = query.getDimensions();
       dimensions = Lists.newArrayListWithExpectedSize(dimensionSpecs.size());
-      dimNames = new String[dimensionSpecs.size()];
+      dimNames = Lists.newArrayListWithExpectedSize(dimensionSpecs.size());
       for (int i = 0; i < dimensionSpecs.size(); ++i) {
         final DimensionSpec dimSpec = dimensionSpecs.get(i);
         final DimensionSelector selector = cursor.makeDimensionSelector(dimSpec.getDimension());
         if (selector != null) {
           dimensions.add(selector);
+          dimNames.add(dimSpec.getOutputName());
         }
-        dimNames[i] = dimSpec.getOutputName();
       }
 
       aggregatorSpecs = query.getAggregatorSpecs();
@@ -378,7 +379,7 @@ public class GroupByQueryEngine
 
                   ByteBuffer keyBuffer = input.getKey().duplicate();
                   for (int i = 0; i < dimensions.size(); ++i) {
-                    theEvent.put(dimNames[i], dimensions.get(i).lookupName(keyBuffer.getInt()));
+                    theEvent.put(dimNames.get(i), dimensions.get(i).lookupName(keyBuffer.getInt()));
                   }
 
                   int position = input.getValue();

server/src/test/java/com/metamx/druid/query/group/GroupByQueryRunnerTest.java

@@ -695,6 +695,41 @@ public class GroupByQueryRunnerTest
     TestHelper.assertExpectedObjects(expectedResults, mergeRunner.run(query), "no-limit");
   }
 
+  @Test
+  public void testGroupByWithNonexistantDimension() throws Exception
+  {
+    GroupByQuery.Builder builder = GroupByQuery
+        .builder()
+        .setDataSource(QueryRunnerTestHelper.dataSource)
+        .setInterval("2011-04-02/2011-04-04")
+        .addDimension("billy")
+        .addDimension("quality")
+        .setAggregatorSpecs(
+            Arrays.<AggregatorFactory>asList(
+                QueryRunnerTestHelper.rowsCount
+            )
+        )
+        .setGranularity(new PeriodGranularity(new Period("P1M"), null, null));
+
+    final GroupByQuery query = builder.build();
+
+    List<Row> expectedResults = Arrays.asList(
+        createExpectedRow("2011-04-01", "quality", "automotive", "rows", 2L),
+        createExpectedRow("2011-04-01", "quality", "business", "rows", 2L),
+        createExpectedRow("2011-04-01", "quality", "entertainment", "rows", 2L),
+        createExpectedRow("2011-04-01", "quality", "health", "rows", 2L),
+        createExpectedRow("2011-04-01", "quality", "mezzanine", "rows", 6L),
+        createExpectedRow("2011-04-01", "quality", "news", "rows", 2L),
+        createExpectedRow("2011-04-01", "quality", "premium", "rows", 6L),
+        createExpectedRow("2011-04-01", "quality", "technology", "rows", 2L),
+        createExpectedRow("2011-04-01", "quality", "travel", "rows", 2L)
+    );
+
+    TestHelper.assertExpectedObjects(expectedResults, runner.run(query), "normal");
+    QueryRunner<Row> mergeRunner = new GroupByQueryQueryToolChest().mergeResults(runner);
+    TestHelper.assertExpectedObjects(expectedResults, mergeRunner.run(query), "no-limit");
+  }
+
   private Row createExpectedRow(final String timestamp, Object... vals)
   {
     return createExpectedRow(new DateTime(timestamp), vals);